Re: [PATCH bpf v2 2/2] selftests/bpf: fix and consolidate d_path LSM regression test

[Matt Bobrowski <mattbobrowski@google.com>]

On Tue, Dec 02, 2025 at 05:30:15PM +0800, Shuran Liu wrote:
> Hi Matt,
> 
> Thanks a lot for the review and for re-sending your Reviewed-by tag.
> 
> In the next version of the series I’ll add your
> 
> Reviewed-by: Matt Bobrowski <mattbobrowski@google.com>
> 
> to the patch that introduces the new selftest, and I’ll also make sure to
> remove /tmp/bpf_d_path_test in the test cleanup path as you suggested.

SGTM.

> I also noticed that the CI is currently failing due to the `#pragma unroll`
> around the loop in prog_lsm(). Would you prefer that I simply drop the pragma
> in the next version, given that the loop bound is small and constant anyway,
> or is there a better way you’d recommend to handle this?

Yeah, I don't think the use of this directive is required here given
the iteration count is tiny. Alternatively, perhaps you could switch
over to using a more BPF verifier preferred alternative
(i.e. bpf_for() or better yet and simpler bpf_repeat())?

> > On Tue, Dec 02, 2025 at 00:59:45AM -0800, Matt Bobrowski wrote：
> > 
> > On Tue, Dec 02, 2025 at 03:54:41PM +0800, Shuran Liu wrote:
> >> Add a regression test for bpf_d_path() when invoked from an LSM program.
> >> The test attaches to the bprm_check_security hook, calls bpf_d_path() on
> >> the binary being executed, and verifies that a simple prefix comparison on
> >> the returned pathname behaves correctly after the fix in patch 1.
> >> 
> >> To avoid nondeterminism, the LSM program now filters based on the
> >> expected PID, which is populated from userspace before the test binary is
> >> executed. This prevents unrelated processes that also trigger the
> >> bprm_check_security LSM hook from overwriting test results. Parent and
> >> child processes are synchronized through a pipe to ensure the PID is set
> >> before the child execs the test binary.
> >> 
> >> Per review feedback, the new test is merged into the existing d_path
> >> selftest rather than adding new prog_tests/ or progs/ files.
> >> 
> >> Co-developed-by: Zesen Liu <ftyg@live.com>
> >> Signed-off-by: Zesen Liu <ftyg@live.com>
> >> Co-developed-by: Peili Gao <gplhust955@gmail.com>
> >> Signed-off-by: Peili Gao <gplhust955@gmail.com>
> >> Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
> >> Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
> >> Signed-off-by: Shuran Liu <electronlsr@gmail.com>
> > 
> > Feel free to add:
> > 
> > Reviewed-by: Matt Bobrowski <mattbobrowski@google.com <mailto:mattbobrowski@google.com>>
> > 
> >> ---
> >> .../testing/selftests/bpf/prog_tests/d_path.c | 64 +++++++++++++++++++
> >> .../testing/selftests/bpf/progs/test_d_path.c | 33 ++++++++++
> >> 2 files changed, 97 insertions(+)
> >> 
> >> diff --git a/tools/testing/selftests/bpf/prog_tests/d_path.c b/tools/testing/selftests/bpf/prog_tests/d_path.c
> >> index ccc768592e66..2909ca3bae0f 100644
> >> --- a/tools/testing/selftests/bpf/prog_tests/d_path.c
> >> +++ b/tools/testing/selftests/bpf/prog_tests/d_path.c
> >> @@ -195,6 +195,67 @@ static void test_d_path_check_types(void)
> >> 	test_d_path_check_types__destroy(skel);
> >> }
> >> 
> >> +static void test_d_path_lsm(void)
> >> +{
> >> +	struct test_d_path *skel;
> >> +	int err;
> >> +	int pipefd[2];
> >> +	pid_t pid;
> >> +
> >> +	skel = test_d_path__open_and_load();
> >> +	if (!ASSERT_OK_PTR(skel, "d_path skeleton failed"))
> >> +		return;
> >> +
> >> +	err = test_d_path__attach(skel);
> >> +	if (!ASSERT_OK(err, "attach failed"))
> >> +		goto cleanup;
> >> +
> >> +	/* Prepare the test binary */
> >> +	system("cp /bin/true /tmp/bpf_d_path_test 2>/dev/null || :");
> > 
> > I'd much prefer if we also cleaned up after ourselves, but it's not
> > that much of an issue I guess.
> > 
> >> +	if (!ASSERT_OK(pipe(pipefd), "pipe failed"))
> >> +		goto cleanup;
> >> +
> >> +	pid = fork();
> >> +	if (!ASSERT_GE(pid, 0, "fork failed")) {
> >> +		close(pipefd[0]);
> >> +		close(pipefd[1]);
> >> +		goto cleanup;
> >> +	}
> >> +
> >> +	if (pid == 0) {
> >> +		/* Child */
> >> +		char buf;
> >> +
> >> +		close(pipefd[1]);
> >> +		/* Wait for parent to set PID in BPF map */
> >> +		if (read(pipefd[0], &buf, 1) != 1)
> >> +			exit(1);
> >> +		close(pipefd[0]);
> >> +		execl("/tmp/bpf_d_path_test", "/tmp/bpf_d_path_test", NULL);
> >> +		exit(1);
> >> +	}
> >> +
> >> +	/* Parent */
> >> +	close(pipefd[0]);
> >> +
> >> +	/* Update BPF map with child PID */
> >> +	skel->bss->my_pid = pid;
> >> +
> >> +	/* Signal child to proceed */
> >> +	write(pipefd[1], "G", 1);
> >> +	close(pipefd[1]);
> >> +
> >> +	/* Wait for child */
> >> +	waitpid(pid, NULL, 0);
> >> +
> >> +	ASSERT_EQ(skel->bss->called_lsm, 1, "lsm hook called");
> >> +	ASSERT_EQ(skel->bss->lsm_match, 1, "lsm match");
> >> +
> >> +cleanup:
> >> +	test_d_path__destroy(skel);
> >> +}
> >> +
> >> void test_d_path(void)
> >> {
> >> 	if (test__start_subtest("basic"))
> >> @@ -205,4 +266,7 @@ void test_d_path(void)
> >> 
> >> 	if (test__start_subtest("check_alloc_mem"))
> >> 		test_d_path_check_types();
> >> +
> >> +	if (test__start_subtest("lsm"))
> >> +		test_d_path_lsm();
> >> }
> >> diff --git a/tools/testing/selftests/bpf/progs/test_d_path.c b/tools/testing/selftests/bpf/progs/test_d_path.c
> >> index 84e1f883f97b..7f65c282069a 100644
> >> --- a/tools/testing/selftests/bpf/progs/test_d_path.c
> >> +++ b/tools/testing/selftests/bpf/progs/test_d_path.c
> >> @@ -17,6 +17,8 @@ int rets_close[MAX_FILES] = {};
> >> 
> >> int called_stat = 0;
> >> int called_close = 0;
> >> +int called_lsm = 0;
> >> +int lsm_match = 0;
> >> 
> >> SEC("fentry/security_inode_getattr")
> >> int BPF_PROG(prog_stat, struct path *path, struct kstat *stat,
> >> @@ -62,4 +64,35 @@ int BPF_PROG(prog_close, struct file *file, void *id)
> >> 	return 0;
> >> }
> >> 
> >> +SEC("lsm/bprm_check_security")
> >> +int BPF_PROG(prog_lsm, struct linux_binprm *bprm)
> >> +{
> >> +	pid_t pid = bpf_get_current_pid_tgid() >> 32;
> >> +	char path[MAX_PATH_LEN] = {};
> >> +	int ret;
> >> +
> >> +	if (pid != my_pid)
> >> +		return 0;
> >> +
> >> +	called_lsm = 1;
> >> +	ret = bpf_d_path(&bprm->file->f_path, path, MAX_PATH_LEN);
> >> +	if (ret < 0)
> >> +		return 0;
> >> +
> >> +	{
> >> +		static const char target_dir[] = "/tmp/";
> >> +
> >> +#pragma unroll
> >> +		for (int i = 0; i < sizeof(target_dir) - 1; i++) {
> >> +			if (path[i] != target_dir[i]) {
> >> +				lsm_match = -1; /* mismatch */
> >> +				return 0;
> >> +			}
> >> +		}
> >> +	}
> >> +
> >> +	lsm_match = 1; /* prefix match */
> >> +	return 0;
> >> +}
> >> +
> >> char _license[] SEC("license") = "GPL";
> >> -- 
> >> 2.52.0
>